The present invention relates generally to methods for growing single crystals of ferroelectric materials which are thermally unstable at temperatures below their melting points. More particularly, the present invention relates to growing single crystals from a melt of the ferroelectric material in a manner such that the material does not thermally decompose and so that a high quality crystal is produced.
Ferroelectric materials which are generally classified as belonging to the KDP family include the following compounds:
KH.sub.2 PO.sub.4 (or KDP), KD.sub.2 PO.sub.4 (or KD*P), RbH.sub.2 PO.sub.4 (or RDP), RbD.sub.2 PO.sub.4 (or RD*P), CsH.sub.2 PO.sub.4, CsD.sub.2 PO.sub.4, (NH.sub.4)H.sub.2 PO.sub.4 (or ADP), (NH.sub.4)D.sub.2 PO.sub.4 (or AD*P), (NH.sub.3 D)H.sub.2 PO.sub.4, (NH.sub.3 D)D.sub.2 PO.sub.4, (NH.sub.2 D.sub.2)H.sub.2 PO.sub.4, (NH.sub.2 D.sub.2)D.sub.2 PO.sub.4, (NHD.sub.3)H.sub.2 PO.sub.4, (NHD.sub.3)D.sub.2 PO.sub.4, (ND.sub.4)H.sub.2 PO.sub.4, (ND.sub.4)D.sub.2 PO.sub.4, KH.sub.2 AsO.sub.4, KD.sub.2 AsO.sub.4, RbH.sub.2 AsO.sub.4 (or RDA), RbD.sub.2 AsO.sub.4, CsH.sub.2 AsO.sub.4 (or CDA), CsD.sub.2 AsO.sub.4 (or CD*A), (NH.sub.4)H.sub.2 AsO.sub.4 (or ADA), (NH.sub.4)D.sub.2 AsO.sub.4 (or AD*A), (NH.sub.3 D)H.sub.2 AsO.sub.4, (NH.sub.3 D)D.sub.2 AsO.sub.4, (NH.sub.2 D.sub.2)D.sub.2 AsO.sub.4, (NH.sub.2 D.sub.2)D.sub.2 AsO.sub.4, (NHD.sub.3)H.sub.2 AsO.sub.4, (NHD.sub.3)D.sub.2 AsO.sub.4, (ND.sub.4)H.sub.2 AsO.sub.4, (ND.sub.4)D.sub.2 AsO.sub.4 and mixtures of these.
Compounds such as KHDPO.sub.4, (NH.sub.4)HDPO.sub.4, etc. or mixtures of deuterated and undeuterated counterparts are also considered part of the KDP family. Single crystals made from these materials are known to have good non-linear optical properties and are, therefore, used extensively in the electro-optical industries.
The above-listed ferroelectric materials which belong to the KDP family are notoriously thermally unstable at temperatures below their melting points. Typically, the materials undergo partial decomposition as they are heated to temperatures near their melting point. Due to this thermal instability, conventional high temperature crystal growth methods, such as the Czochralski and Bridgman techniques where crystals are grown from their respective melts, have not been developed. Instead, the crystals are grown at much lower temperatures from aqueous solutions by solvent evaporation.
The solvent evaporation method is the conventional procedure used in large-scale production of these crystals. This method is a slow process, with growth rates on the order of one millimeter per day being typical. The quality of the crystals grown by evaporation from solution suffers from microscopic solvent inclusions. Further, some optical and electro-optical properties of the crystals grown from solution will vary depending upon the amounts of potassium carbonate (K.sub.2 CO.sub.3) and/or phosphoric acid (H.sub.3 PO.sub.4) additives added to the growth solutions. These additives are conventionally used in different amounts by various commercial manufacturers to control solution pH and enhance crystal growth.
It would be desirable to provide a relatively quick method for preparing quality crystals from materials belonging to the KDP family which does not involve the relatively slow growth of crystals from aqueous solutions. It would further be desirable to provide a method which produces crystals having uniform optic and electro-optic properties and which do not have microscopic solvent inclusions.